Assuming "your_view" has a "foo" argument, using argument_picker, you could easily setup this argument's default value with:

<?phpreturn arg_picker_get_view_argument("foo");?>

which would return "4" (for the example above) or "all" (when the argument is not present).

Basically it's just an alternative to an url querystring with Drupal-specific add-on features, which comes extremely useful when implementing progressive filters (like a faceted search) through views but it also comes very handy for retrieving arguments in dynamic PHP blocks, panel panes, custom modules or wherever you want.